In wireless communication systems, portable or mobile subscriber units communicate with a centrally located base station within a cell. The wireless communication systems may be a CDMA2000, GSM or WLAN communication system, for example. The subscriber units are provided with wireless data and/or voice services by the system operator and can connect devices such as, for example, laptop computers, personal digital assistants (PDAs), cellular telephones or the like through the base station to a network.
Each subscriber unit is equipped with an antenna. To increase the communications range between the base station and the mobile subscriber units, and for also increasing network throughput, smart antennas may be used. Smart antennas may also be used with access points and client stations in WLAN communication systems. A smart antenna includes a switched beam antenna or a phased array antenna, for example, and generates directional antenna beams.
A switched beam antenna includes an active antenna element and one or more passive antenna elements. Each passive antenna element is connected to a respective impedance load by a corresponding switch. By selectively switching the passive antenna elements to their impedance load, a desired antenna pattern is generated. When a passive antenna element is connected to an inductive load, radio frequency (RF) energy is reflected back from the passive antenna element towards the active antenna element. When a passive antenna element is connected to a capacitive load, RF energy is directed toward the passive antenna element away from the active antenna element. A switch control and driver circuit provides logic control signals to each of the respective switches.
For a switched beam antenna comprising an active antenna element and two passive antenna elements, for example, there are four different switching combinations for selecting a desired antenna beam if the switch is a single pole double throw (SPDT). Each switching combination corresponds to a different antenna beam mode, and consequently, the input impedance to the active antenna element changes between the difference modes. The efficiency of the smart antenna varies as the input impedance varies.
Similarly, in a phased array antenna, when the relative phases fed to the respective antenna elements are changed, the input impedances also vary. The phase changes are integral to the beam scanning and adaptive beam forming of a phased array antenna. This makes it difficult to match the input impedances of the various modes. To obtain a reasonable match for required beam shapes and positions, dynamic matching circuits are often used, which further add to the complexity and cost of a phased array antenna.